The plasma processes in microwave discharges are based on electrons, because heavy ions are not able to follow changes of the microwave field. The microwave plasma might be produced by: (i) anisotropic generation, and (ii) an anisotropic generation in a magnetic field. The isotropic generation represents simple absorption of the microwave energy in the plasma without any preferential directions defined by external forces. The isotropic plasma has an upper electron density limit, so called cutoff density, which depends linearly on the square of the generator frequency. The most common anisotropic generation is the Electron Cyclotron Resonance (ECR). There the microwave power is absorbed in the plasma in a magnetic field having induction B=Bce=2πm/e, where f is the frequency of the generator (typically 2.4·109 s−1), m and e are the electron mass (9.1·10−31 kg) and electron charge (1.6·109 As), respectively. The value of the ECR field for typical microwave generators is Bce=8.57·10−2 Tesla. In the anisotropic microwave plasmas the plasma electron density may even exceed the cutoff density. The plasma density in the microwave plasma is typically high (≧1010 cm−3), particularly in low pressure ECR plasmas, but the energy of ions is often insufficient. Contrary to this, in a direct current (DC) generation, or at lower frequencies, for example at radio frequency (RF) generation (order of 106-107 Hz) applied through an arbitrary RF electrode, the ions may follow the generating field and gain a sufficient energy. However, a high generation frequency in the microwave case (order of 109 Hz) can provide higher plasma density than any lower frequency generation at the same power, because of higher cutoff density allowed. Therefore an additional DC or RF bias applied to sample holders or auxiliary electrodes is often necessary to increase ion energy in the microwave systems. The efficiency of the microwave generation is very sensitive to the geometry of the launching system. The conventional electrode less generation of the microwave plasma doesn't need metal electrodes that may cause metal contamination. But such systems often require additional electrodes for ion energy control, anyway. Moreover, because of a possibility of coating dielectric windows at microwave inlets by absorbing films, the windows require special arrangement with protecting film depositions in a technology using PE CVD (Plasma Enhanced Chemical Vapor Deposition).
The microwave devices of different constructions have been used for different surface treatments (see for instance M. Moisan and J. Pelletier, eds.: “Microwave Excited Plasmas”, Elsevier, Amsterdam, 1992). During last decade the microwave plasma is frequently used for deposition of carbonaceous films, like diamond or carbon nitrides. In U.S. Pat. No. 4,898,118 the generation of the microwave plasma is fulfilled in a reaction vessel disposed to penetrate through the rectangular wave-guide. In U.S. Pat. No. 4,940,015 the reactor for diamond film synthesis is based on a tunable evacuated microwave cavity adjacent to the rectangular wave-guide. The coupling of the microwave power is fulfilled by an antenna inside the wave-guide and outside the low pressure region directing the microwave power into the cavity through the dielectric window positioned in a bottom side of a particularly designed cylindrical part of the cavity. In U.S. Pat. No. 4,958,590 the generation of the microwave plasma is fulfilled inside a reaction tube of particular design located inside a wave-guide of specified length. The plasma is generated in a travelling wave mode. A device with ECR microwave plasma is claimed in U.S. Pat. No. 4,915,979. In this patent the dimensions of reaction chamber are optimized with respect to Larmor radii of electrons, so that the spatial uniformity of the plasma electron density can be improved. In a Swedish Patent Application 9302222-6 a unique system was described for the isotropic microwave plasma generation. The system is based on a plasma slab generated by surface waves and used as an antenna for a subsequent generation of a bulk microwave plasma in resonator. An additional electrode in this system allows control of both the current to the substrate and coupling of the plasma antenna with the resonator (see e.g., Bárdos et al, J. Vac. Sci. Technol., 1995). A simple electrode generation of the microwave plasma has been recently reported for PE CVD of C—N films (Bardos et al, Proc. SVC Tech. Con. 1999). The system combines efficient low power generation with a possibility of restriction of the plasma zone at the processed surface and easy application of auxiliary fields. In the described present art the electrodes in the microwave plasma were used only either as microwave launching antennas or as auxiliary applicators for additional electric fields. Additional fields in microwave plasma may be used for generation of independent plasma or plasma dependent on pre-ionization from the original microwave plasma. These combinations may lead to more suitable advanced processing plasma, denoted usually as a “dual plasma” or “hybrid plasma”. Example of a hybrid plasma generated in an anisotropic (magnetized) microwave plasma combined with DC and AC fields applied by a set of electrodes is claimed in Japanese patent No. 01191779 A by F. Kanji (1988). A typical example of dual plasma generated in an isotropic microwave plasma is claimed in U.S. Pat. No. 4,691,662 by T. A. Roppel et al. (1986). Here a disk microwave plasma acts as a source of excited ion and free radical species and electrons for the second plasma which is hybrid in that it contains species from both microwave and DC (or RF depending bias) excitation through metal plate means. The system may work also with an anisotropic plasma in the ECR mode.
Contrary to “soft” microwave plasmas for chemistry-based treatments, for instance plasma etching or PE CVD, the Physical Vapor Deposition (PVD) of films requires presence of a solid target (usually cathode) and either high ion energies (for sputtering) or large electron (or ion) current for heating (evaporation) of the target. Very efficient “electrode-based” discharges for surface treatment are generated by hollow cathodes. The cathode is connected to a negative pole of a DC generator and the positive pole is connected to a suitable counter anode. Depending on the DC power the hollow cathode discharge can be excited in a glow regime or in an arc regime. The principle of the hollow cathode discharge is based on its suitable geometry, where an electron emitted from one cathode wall interacts with an equivalent electric field with opposite orientation at the opposite wall. Thus the electrons may oscillate between inner walls of the hollow cathode and substantially enhance the ionization of the present gas, or metal deliberated from the cathode wall. Since 1983 the hollow cathode glow discharges have been generated also by alternating currents (AC). Typical frequency of AC generators for this purpose is between 105 s−1 and 108 s−1. The anode in the RF generated hollow cathodes is the RF plasma itself (a virtual anode), in contact with the real counter electrode connected to the RF generator (Bardos et al., J. Non Cryst. Solids 97/98, 281 (1987)). Effects of additional magnetic fields have been found in hollow cathodes, see e.g. review by K. H. Schoenbach, invited paper at ICPIG 21, Bochum 1993, Proc. III, pp. 287-296. A focused magnetic field was used in an apparatus for generation of a linear arc discharge for plasma processing (LAD) by Bardos et al. in a Swedish Patent Application 9403988-0 (U.S. Pat. No. 5,908,602). In this apparatus a pair of electrode plates placed opposite to each other forms parallel-plate hollow cathode, negative with respect to the surrounding plasma. The magnetic field perpendicular to the cathode plates and located close to the cathode outlet facilitates the hollow cathode discharge between the plates in the outlet slit. The hot zones are formed at both plates along the outlet slit due to an enhanced ion bombardment of the plate surfaces. The magnetic field geometry in the LAD system is stationary in both time and space. In the Swedish Patent Application 9704260-0 by Barankova et al. a plasma processing apparatus with rotary magnets for obtaining an adjustable time variable magnetic field has been claimed. The rotary permanent magnet systems, comprising individual permanent magnets with maximum magnetic induction more than 10−1 Tesla, may be installed along the outlet slit of the linear hollow cathode for better control of the hollow cathode discharge.
Because of high production of electrons even in glow regimes the hollow cathodes have been used since 1971 as both an electron source and the working gas ionization source in plasma processing devices for plasma assisted evaporation. The hollow cathode may enhance sputtering rate of magnetrons when used as an auxiliary source of electrons close to the target erosion zone (U.S. Pat. No. 4,588,490 1986 by J. J. Cuomo et al.). An another suitable application of the hollow cathode is its combination with an arc evaporator (see A. Lunk, Vacuum, 1990). These applications might be considered as an example of hollow cathode assisted hybrid plasma. However, no devices utilizing both the hollow cathode discharge and the microwave plasma simultaneously in a suitable hybrid system have been described yet. There are also no works or results yet reporting about combinations of the magnetized hollow cathodes with other plasma systems.